The present invention relates to a process for manufacturing a tire for vehicle wheels and more particularly to a tire designed to be mounted on heavy transport motor-vehicles.
As used throughout this specification, the word "axial" refers to a direction generally parallel to the axis of rotation of the tire and the word "radial" refers to a direction generally perpendicular thereto.
The present tire is provided with a carcass comprising two beads each defined along an inner circumferential edge of the tire for securing it to a corresponding mounting rim. The carcass also comprises a pair of annular reinforcement cores, usually referred to as bead cores, each fitted in one of the beads, each bead core consisting of a plurality of coils of bare metal wire disposed in axial side by side relation and radially superposed, the wires being tightly packed together so as to form bead core having a transverse polygonal cross section, the bead core being then enclosed in a covering structure. There is a filler or apex strip of elastomeric material extending along the outer circumferential edge of the bead core and radially tapering away from the tire axis. In addition, the carcass has a supporting structure consisting of at least one ply of rubberized fabric reinforced with textile or metal cords, extending axially from one bead to the other and having its edges folded back about the bead cores and the fillers.
It is known that the engagement between a tire and the respective wheel rim is generally achieved by means of areas extending circumferentially within the tire and generally referred to as "beads". Generally inserted in each tire bead is a so-called "bead core" performing the double function of giving the appropriate non-deformability character to the bead when the tire is being used and holding the end edges of the carcass ply or plies which are folded back around the bead core itself. The above-mentioned elastomeric filler is of substantially triangular section and has the function of stiffening the tire bead and filling the space left empty between the carcass plies the folded or turned up end portions thereof.
Each bead core is an annulus formed by a plurality of wire coils usually of metal material, disposed in axial side by side relation and radially superposed so as to give the desired geometrical configuration to the bead core when seen in cross section.
In tubeless tires, that is those without an inner tube, the bead, and specifically its reinforcement bead core, constitute a particularly critical element in the tire.
In fact, due to the absence of an inner tube, a rim of one piece construction, that is the so-called "grooved rim" or drop center type is required to act as an air-tight coupling between the tire beads for the purpose of maintaining the inflation pressure in the tire.
The rim is provided with bead retaining radially outwardly extending flanges. It is possible to carry out the mounting of a tire on the rim and the dismantling therefrom by making the tire bead override the retaining flanges by means of a slight oval conformation given to the bead on one side with on the diametric opposite side the bead is in a drop center position or well formed in the axial center part of the wheel rim.
To achieve air-tightness between the tire and the rim, the rim is provided with bead seats which are radially inclined towards the outside by an angle of 15.degree. relative to the tire axis; during the tire inflation the inflation pressure urges the beads to slide axially towards the outside on said inclined surface of progressively increasing diameter and, since the reinforcement bead core is circumferentially inextensible by itself, as a result this sliding brings about a tight fit between the bead and the rim, so that the elastomeric material interposed between the bead core and the bead housing is compressed, thereby achieving air tightness between the tire and the rim.
However, to properly achieve this tight fit a bead core is necessary in which its surface facing the rim is substantially parallel to the base of the bead, that is, also inclined by 15.degree. relative to the rotational axis of the tire and axially extending over at least 50% of the axial width of the bead itself.
In order to comply with this requirement, bead cores having a polygonal, rhomboidal or hexagonal cross section have been long provided and they consist of a pack of coils of bare metal wire, disposed in axial side by side relation and radially superposed, such as for example the bead core disclosed in the U.S. Pat. No. 3,949,800, the disclosure of which is hereby incorporated by reference.
This type of bead core is very flexible since the coils of bare wire can be easily displaced relative to one another and therefore it can be easily made oval so that the assembling and disassembling of the tire from the rim can be easily achieved.
On the other hand, however, these bead cores, due to the fact that they are made of coils of bare wire, have some serious drawbacks as compared with the usual bead cores made of rubberized wire, which drawbacks reduce the useful lifetime of the tire.
First of all they have a weak shape stability and specifically weak torsional resistance so that during the molding and vulcanization step of the tire, under the thrust of the vulcanization pressure and the pulling action exerted by the carcass plies, their polygonal cross section loses its starting geometrical form which should be rigidly shaped and take a different form which is basically round, in particular at the side facing the rim, thereby diminishing its axial width and varying the inclination thereof relative to the desired one, that is 15.degree..
As a result of this deformation the tire bead is unable to withstand the stress to which it is submitted in use over an extended period of time; in particular, also due to the reduced height of the rim flange, the tire bead during use begins to rotate about its radially inner and axially outer edge thereby causing detachment from the bead base resting on the rim bead seats, starting from a relatively small point and progressively going on over an increasingly greater portion with the result that its structural strength is impaired.
A second drawback originates from the fact that the rubber/metal bonding on the bare metal, as in the case of the concerned bead core, is difficult to obtain and does not possess high mechanical strength, so that, as time goes by, breakage of the bonding interface can take place in the tire in use and, as a result, the bead core can be separated from the bead, which then involves the necessity to replace the tire within a short period of time.
In order to avoid the first drawback, that is bead core deformation during the vulcanization step, in accordance with the known art a bare bead core is incorporated into a shell of elastomeric material and a partial vulcanization of the assembly is carried out before applying the bead filler and inserting the bead core in the carcass of the tire being manufactured.
However while this partial vulcanization on the one hand exerts a sufficiently stiffening action on the bead core which is therefore capable of going through the tire vulcanizing step without undergoing important deformations, on the other hand the partial vulcanization adversely affects not only the rubber/metal bonding between the shell and the bead core but also the rubber/rubber bonding between the partial vulcanized shell surface and the other bead elements still in a raw (unvulcanized) state, thereby further impairing a situation which is already critical.
These types of bonding are not even improved by the final vulcanization step, since the bonding interfaces have already reached stabilization during the previous partial vulcanization step.
In particular, rubber in the raw (unvulcanized) state, as is known, has some adhesiveness. This adhesiveness is used for joining together the different rubber parts constituting the tire, not only mechanically but also from a molecular point of view during the final vulcanization step. Unfortunately in this case the adhesiveness of the rubber shell covering the bead core is reduced, sometimes even to zero, due to the preceding partial vulcanization step and therefore it substantially reduces the capability of the rubber shell to be properly coupled and bonded with the elastomeric material of the bead filler and the other surrounding rubberized fabrics.
As a result, it is necessary to submit this rubber shell to a solutioning step, in order to give it back, at least partly, the adhesiveness features that it has lost during the partial vulcanization.
In order to obviate this requirement as much as possible, attempts have been made to improve this bonding, in accordance with the teachings contained in British Patent 2,064,442 in the name of the same Assignee (the disclosure of which is hereby incorporated by reference), consisting of covering the rubber shell with a layer of different rubber, optionally incorporating a reinforcement cord structure made of a heat-shrinkable material such as nylon having a different vulcanization rate. In particular, the rubber in the shell, that is in contact with the bead core, has a higher vulcanization rate whereas the rubber in the outer layer has a lower vulcanization rate. Therefore, when the covered bead core is submitted to a partial vulcanization step, the outermost layer substantially remains in a raw state and, as such, is capable of adhering to a greater extent to the surrounding rubber parts, which are in a raw state also, during the final vulcanization step.
Alternatively, in accordance with the teachings of the Italian Patent No. 1,151,359 in the name of the same Assignee (the disclosure of which is hereby incorporated by reference), the shell has been replaced with a rubberized fabric strip reinforced with textile cords preferably of nylon; then a second rubberized fabric covering reinforced with cords made of heat-shrinkable material preferably consisting of a fabric strip coiled about the taped bead core is applied after the partial vulcanization. In this manner, when the tire is submitted to the final vulcanization step the nylon cords of the second covering become shorter thereby exerting a strong pressure on the first covering and, as a result, the friction between the two concentric coverings brings about a chemical cooperation for the achievement of a sufficient degree of bonding.
On the other hand it is apparent that the shape stability and torsional strength of the bead core section is not at all increased.
These processes solve the problem connected with the rubber/rubber bonding at least partly but do not succeed in substantially improving the weak bonding created between the bead core metal and its shell during the partial vulcanization step.
As regards the increase of the torsional strength of the metal bead core in itself, sections of the wire (some of which are disclosed in the above mentioned U.S. Patent) have been studied that interact with one another so as to make the bead core cross section steadier.
In this connection the Assignee itself has recently offered a brilliant solution to the problem, in accordance with the U.S. Pat. No. 5,007,471, by providing a bead core in which coils made of wire having a hexagonal flattened or elongated cross section are mutually interfitted with coils disposed in side by side relation therewith, with a half-coil staggering, thereby forming a closely packed section that is practically not-deformable both in the axial and radial direction, which will ensure the bead core a very high transverse stability and torsional strength.
In greater detail, in an unmounted tire, that is during the assembling and disassembling step of the tire with and from the rim, the bead wire coils can move with respect to one another giving a high flexibility and deformability to the bead core whereas under the effect of the inflation pressure, that is when the coils are in tension, the bead core becomes stiff, thereby acquiring a practically not-deformable cross section.